1. Field of the Invention
The present invention relates, in general, to automatic transmissions for automobiles and, more particularly, to an ATF bypass device for such transmissions capable of allowing ATF (automatic transmission fluid) to selectively bypass an ATF cooler when the ATF temperature is substantially low enough to allow the ATF to be free from any requirement of cooling, thus allowing the ATF at an appropriate temperature to selectively and quickly circulate in an automatic transmission.
2. Description of the Prior Art
As is well known to those skilled in the art, ATF (automatic transmission fluid), which is used as actuating oil in automatic transmissions for automobiles, varies in its viscosity in accordance with its temperature. Such a variation in the ATF's viscosity may have a negative effect on the operational performance of automatic transmissions.
Particularly, when the temperature of the ATF at the time of starting an engine is not fixed due to variable climatic temperatures it may vary in its viscosity. Therefore, it is impossible to expect uniform operational performance of an automatic transmission or to expect uniform output power of an engine when the engine is started.
Meanwhile, when the engine runs at a high rpm (revolutions per minute) or runs for a lengthy period of time, the temperature of the ATF rises resulting in the viscosity of the ATF being reduced, thus causing the power transmission efficiency of the automatic transmission to be reduced. In order to overcome the above problem caused by the high temperature ATF, the ATF temperature may be reduced by an oil cooler (ATF cooler) when the ATF temperature rises excessively.
FIG. 1 shows an automatic transmission with a typical ATF cooling device. In the operation of the automatic transmission 10, ATF is repeatedly discharged from the transmission 10 into an ATF cooler 20 through an outlet port 11 during the circulation of the ATF in the transmission 10. The ATF cooler 20 cools the ATF, thus reducing the ATF temperature prior to returning the ATF into an oil tank 15 of the transmission 10 through a return port 12.
However, the above ATF cooling device is problematic in that the ATF has to repeatedly pass through the cooler 20 regardless of the ATF temperature. The cooling device thus prevents the ATF temperature from quickly warming and rising to an appropriate point when starting the engine in low temperatures. In such a case, the ATF has an undesirably poor fluidity for a time, thus generating a shifting shock and reducing the operational performance of the engine, with the engine being started in low temperatures.